The present invention relates to an ultrasound diagnostic apparatus for reconstructing an image based on echo signals obtained by scanning a cross-section of a subject with an ultrasound beam
The ultrasound spectrums are distorted by various causes, for example, attenuation. The attenuation has a frequency dependence Specifically, as shown in FIG. 1, the attenuation does not constantly act on the every frequency component. In other words, the attenuation acts on the high frequency components strongly and the low frequency components weakly. The longer the propagation distance becomes (i.e., as the reflecting depth becomes deeper), the larger the distortion of the spectrum becomes (i.e., the depth dependence increases).
Conventionally, the distortion of the spectrum due to attenuation was corrected by enhancing the high frequency components having a large attenuation.
However, in the conventional correcting method, the correction accuracy was low. The reason can be explained as follows.
In the conventional ultrasound diagnostic apparatus, a piezoelectric element is used as an interface between an electrical signal and an ultrasound signal. The piezoelectric element receives a voltage causing it to oscillate at high frequency and emit ultrasound. The spectrum of the emitted ultrasound is determined by convolution of the spectrum of the high frequency voltage and the sensitivity characteristic of the piezoelectric element. The sensitivity characteristic of the piezoelectric element is defined as a frequency dependence in which the transform efficiency to the ultrasound from the electrical signal is changed according to the frequency.
Regarding the sensitivity characteristic of the piezoelectric elements the number of cases in which a relatively simple shape is shown as FIG. 2 (a sine curve is drawn) is small. Generally, the complicated shape is shown. Therefore, the transformation to the ultrasound signal from the electrical signal deforms the spectrum of the ultrasound The spectrum also deforms due to the above-mentioned attenuation and the transformation to the electrical signal from the echo.
Due to this deformation complicated distortion of the spectrum cannot be sufficiently corrected by simply enhancing the high frequency components. This problem becomes significant in the probe, which has been recently developed in consideration of a Doppler mode which deals with a blood flow echo whose amplitude is considerably smaller than a tissue echo.
In the Doppler mode, the low frequency ultrasound whose attenuation is smaller is advantageous. The sensitivity peak of the probe tends to be shifted to the low frequency to effectively generate the ultrasound with a low frequency. In the B-mode, unlike the Doppler mode, a spatial resolution is regarded as important. The spatial resolution generally is improved as the frequency becomes high. As a result, in order to achieve the high-sensitivity Doppler mode and the high-spatial resolution B-mode, a situation in which a probe having a sensitivity peak in a low frequency and is driven by a voltage having a high frequency may occur. This state enlarges the distortion of the spectrum in the B-mode, so that the correction accuracy of the spectrum is largely lowered.